1. Field of the Invention
The present invention relates to a novel process for the preparation of toughened thermoplastics by mechanically dewatering a water-moist elastomer component A containing up to 50% by weight of residual water and mixing the dewatered elastomer component A' with a thermoplastic polymer B in an extruder.
The present invention furthermore relates to special embodiments of the stated process, including those using certain components A and B, and to molding materials prepared by the process and to the use of the molding materials for the production of films, fibers and moldings.
2. Description of the Related Art
Particulate rubbers, which may be grafted or ungrafted, are frequently used as elastomer components for toughening thermoplastics or other plastics. Such rubbers are usually prepared in aqueous systems, for example by emulsion or suspension polymerization. The particles formed in the suspension polymerization or precipitated in the emulsion polymerization by the addition of a coagulating precipitating agent are generally washed with water and dewatered by a suitable dewatering method, such as sieving, filtration, decanting, centrifuging or partial drying at elevated temperatures, for example by means of a flow dryer or a spray dryer. Partially dewatered products are obtained in every case.
Frequently used graft rubbers are, for example, polybutadiene grafted with a styrene/acrylonitrile copolymer (SAN) and poly-n-butyl acrylate grafted with such a copolymer, or rubbers composed of a plurality of graft stages based on butadiene, styrene, n-butyl acrylate, methyl methacrylate and/or acrylonitrile.
The residual water content of the rubber obtained after the partial dewatering is up to 50% by weight and is usually removed by energy-consumptive drying. The dried rubber obtained as a powder is finally incorporated, with melting, into the thermoplastics present as powder or granules, the end product being formed. The rubber powder tends to undergo spontaneous ignition during drying and incorporation into the thermoplastics, owing to the fine dust content.
According to a proposal described in DE-A-20 37 784, partially dewatered graft rubber can be introduced into an SAN melt with evaporation of the water and a thermoplastic containing this graft rubber can be obtained. This process requires the use of a relatively large amount of electrical energy.
EP-A 534 235 describes a process for the preparation of toughened thermoplastics by incorporating mechanically partially dewatered rubber into thermoplastic at above the softening point of the thermoplastic, the incorporation being effected in a main extruder and the partial dewatering of the rubber in a side extruder located laterally with respect to the main extruder. The residual water remaining in the rubber is removed as steam during the incorporation, through devolatilization orifices present before and after the feed point.
The disadvantage of this process is the necessity of having to operate two extruders to prepare the impact-resistant thermoplastic. Moreover, the dewatering of the rubber in the side extruder is not complete, so that a large amount of water has to be evaporated in the main extruder.
U.S. Pat. No. 5,151,026 describes an extruder in which comminuted and washed plastics wastes whose water content is up to 50% by weight are dewatered. For this purpose, the extruder, which otherwise has a right-handed thread in the usual manner, contains short sections having a left-handed thread. The application U.S. Pat. No. 5,232,649 arising out of this U.S. publication describes the corresponding process.
Japanese publication JP 22 86 208 describes a twin-screw extruder for dewatering thermoplastic molding materials, whose right-handed screws each have two sections possessing left-handed threads. The water emerges as liquid through Seiher barrels--sieve-like inserts in the extruder barrel--and as steam through devolatilization orifices. However, the Seiher barrels tend to become blocked by emerging polymer material, as is described, for example, in DE 15 79 106 for the dewatering of synthetic rubber.
JP-A 1/202 406 likewise describes a process in which moist rubber-like polymers are initially partially dewatered in a zone provided with Seiher barrels in an extruder and thereafter the residual water is removed in a devolatilization zone under atmospheric pressure and three subsequent devolatilization zones under reduced pressure. This process also includes a complicated devolatilization zone at reduced pressure, in addition to the susceptible Seiher barrels.
U.S. Pat. No. 4,802,769 describes an extruder in which an aqueous slurry of a rubber polymer and a styrene/acrylonitrile copolymer are processed to give a thermoplastic. The water emerges as a liquid through Seiher barrels and as steam through a three-stage devolatilization. In addition to the Seiher barrels which become blocked, disadvantages are that the extruder section provided with Seiher barrels is heated and that multiple pressure build-up occurs in the devolatilization section as a result of retarding elements, with the result that the polymer material is subjected to considerable thermal and mechanical stress.
DE 30 26 842 describes a twin-screw extruder for the devolatilization of thermoplastics, in whose devolatilization and discharge zone the screw roots have a smaller diameter, and the twin-screw barrel a larger diameter, than the preceding zones. Small amounts of water are added to the thermoplastic. In the wider devolatilization zone operated under reduced pressure, steam is formed with the foaming of the thermoplastic and entrains the residual monomers to be removed. As a result of the foaming of the thermoplastic melt, the operation under reduced pressure is relatively susceptible to faults.